A series of laboratory experiments for mixing between the more viscous upper layer and the less viscous lower layer has been performed to study how the less viscous D″ mixes with the overlying more viscous mantle. Experimental results show that the style of entrainment when the more viscous layer overlies the less viscous layer is different from the opposite case. In the latter case the interface between the two layers moves upward, indicating that the more viscous lower layer asymmetrically entrains the less viscous upper layer. However, in the former case, an interfacial layer develops between the two layers and the interface does not move, indicating that the volumetric ratio of the two layers remains unchanged. It has been suggested that the compositional difference between mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) originates from the entrainment of D″ by the mantle plumes. The experimental results suggest that the mantle plume entrains only the interfacial layer, thus modifying the composition of OIB. The volume of D″ would remain unchanged since D″ is formed.